1. Field of the Invention
The present invention relates to a collapsible or extendible linkage suitable for use with a spring centered hydraulic valve and operator's control level in material handling equipment.
2. Description of the Prior Art
In equipment energized by a hydraulic circuit it is often necessary or desirable to arrest the motion of a hydraulically actuated element in one movement direction while permitting movement to occur in another direction. Such a situation can occur when the movable element approaches a limit position as for example, a movable hook block on a crane having an extendible boom and an operator controlled hydraulic motor for driving the cable drum of a winch. If an operator mistakenly hoists the cable too quickly, the hook block will contact the block in the boom head causing possible damage to components, and/or the load to slip from the hook block. A similar result occurs if without simultaneously paying out the cable, the operator extends the boom or pivots upwardly or downwardly a boom of the type having a winch mounted remotely therefrom. In any case, it is desirable to quickly disable the hydraulic component causing action in the direction that would provide this condition and thereby avoid severe damage to the crane. Apparatus which provides such operation is commonly called a "two blocking prevention device".
The foregoing is conventionally accomplished by adding or modifying components in the hydraulic circuitry for the motor. These components are responsive to the presence of the element in the limit position to stop or divert the flow of hydraulic fluid energizing the motor or reduce its pressure to prevent further movement of the element toward the limit position while permitting movement away from the limit position. However, the addition or modification of the necessary hydraulic components adds undesirable expense and complexity to the hydraulic circuitry.